Converting Solar Energy into Power - Photovoltaic Cells

My last post discussed how energy is created in the sun and arrive on earth via packets of energy called photons; it also discussed the amount of sunlight power that falls on the earth, also called irradiance. This post will discuss one of the ways in which we can convert the sunlight into usable power.

One of the ways it can be converted to usable power is via the photovoltaic effect. It was noted that semiconductor materials like selenium release small amounts of electricity when exposed to sunlight. When photons strike these materials, their energy is absorbed and transferred to electrons within the materials. With this energy, the electrons are able to break free from their atoms and flow as in an electric current. Further research into material science resulted in using silicon doped with impurities such as phosphorous and boron.

Adding these impurities to silicon required less energy from the photon for the electron to break free. Silicon doped with phosphorous forms a negative or n-type semiconductor material and silicon doped with boron forms a positive or p-type material. Placing an n-type material in close proximity to a p-type material creates an electric-field at their junction. If free electrons and holes happen to get close to the electric field, the field will send the electrons from the n side to the p side and positive charges or holes from the p side to the n side.

If we add leads to either sides of the semiconductor material and connect them to a load, a circuit is created and electrons can flow through the circuit. The electron flow provides the current, and the cell's electric field results in a potential difference or voltage between the two material types. PV-cells develop direct-current or dc voltage.

The original PV-cells were made from materials such as selenium. These cells were only 1% efficient, meaning only 1% of the sunlight that fell on the material was converted into power. Research using doped silicon yielded cells with efficiency as high as 6%. Current manufactured cells have efficiencies in the range of 14 – 18% and laboratory tests on new cell-designs have shown efficiencies as high as 42%.

A standard solar panel is made from interconnected cells housed in a metal frame and protected with anti-reflective glass; they are typically 1200mm by 600 mm in size. A standard condition for testing solar panel rated power is under an irradiance of 1000W/m², at 25 ºC and a sunlight attenuation factor of AM1.5. This represents the typical conditions in the 48 contiguous United States at noon in the spring and autumn with the solar cell aimed directly at the sun. A panel rated at 14% efficiency is rated to deliver approximately 100W under the standard test conditions.

Currently, the most prevalent material used to make solar cells is crystalline silicon. Silicon is a well-understood semiconductor material with good stability, physical, electrical and chemical properties. Furthermore, silicon cells have benefited from the enormous economies of scale achieved from the semiconductor and microelectronics industry. However, the manufacturing process used to make silicon wafers is complex, which makes them expensive in solar power applications. Furthermore, demand for silicon from the microelectronics and semiconductor industry has driven up material costs.

There is considerable research and development into so called thin-film PV-cells, which use less silicon material. Some of these new types of thin-film PV-cells include CdTe (cadmium telluride), a-Si (amorphous silicon) and CIGS (copper indium gallium selenium). Solar panels formed from these materials have efficiencies in the 11 – 12.2% range, which are lower than that of silicon based panels; however, are much cheaper to manufacture and have higher manufacturing yields. Efficiency advances through continued investment in R&D and in manufacturing techniques could get efficiencies that are comparable with silicon based technologies over time.

Solar Energy is poised for high growth especially with issues related to climate change, a strong desire to diversify sources of energy and from national security concerns. In my next post, I'll continue discussing photovoltaic power and applications for it.  

Getting Power from the Sun

I wanted to learn more about solar energy and I decided to start from the sun because that's where it all starts. The sun, our star is central to all life on Earth! Current theories state that the sun and everything else in the solar system formed from the leftover clouds of gas and dust from a massive star that went supernova. The cloud was mostly hydrogen and helium with trace amounts of other materials.

The sun formed from the coalescing of the materials in the clouds due to their mutual gravitational attraction. After a period of time as the proto-sun continued accumulating more material, it started to collapse under its own weight and build up pressure at the core. As the pressure increased, the temperature at the core started to rise. The pressure at the core is 340 billion times that of the earth at sea level and temperature is approximately 13,600,000 ºC. The present state of the sun is such that the inward pressure of all the material in the outer layers is matched by the outward pressure at the core.

The sun is enormous: it's diameter is approximately 864 million miles and equal to about 109 earths; it's volume is equal to approximately 1,300,000 earths. It is so massive that it comprises approximately 99.9% of the total mass of the solar system including that of the planets, moons, proto-planets, asteroids and comets. It is comprised of about 75% hydrogen, 24% helium and small quantities of other elements like oxygen, neon, iron, carbon and silicon.

At the very center of the sun, the pressure and temperature conditions are favorable for nuclear fusion to occur. Nuclear fusion is the process by which atoms are able to overcome their repulsive electrostatic force and bind to another atom by their attractive nuclear force and release tremendous amounts of energy in the process. The only place this can happen is at the core of stars. 

In this process the mass of four hydrogen atoms fuse into a single helium atom and release roughly 0.7% of the combined mass as energy per Einstein's famous equation E=mc², where E is the energy released, m is the mass lost in the conversion process and c is the speed of light in a vacuum. In one second, the sun converts 600 million metric tons (tonnes) of hydrogen into 595.8 million tonnes of helium; the missing 4.2 million tonnes are converted into energy. Every second, the sun releases approximately 386x10²⁶W of power; the sun releases more energy in one second than used by man in our entire existence!

In the mass-energy conversion process at the core, photons or "packets of energy" are released. It takes anywhere from 10,000 to 170,000 years for the photons to get from the core to the outer edge of the sun called the corona. This is due to the photons continuously being deflected from their path by electrons. Once free of the corona, the photons reach the earth, about 93 million miles away, in about 8.3 minutes. The photons are received by the earth as light and heat.

The sun also emits charged particles (electrons and protons) through its corona as the solar wind. Every second a million tonnes of these charged particles are emitted by the sun as the solar wind; we detect these particles as they interact with the earth's magnetic field as polar auroras or the northern and southern lights. The matter-energy conversion process also release neutrinos, inert particles that aren’t affected by and pass right through matter.

The sun has so much mass that even with the tremendous mass loss through the matter-energy conversion process and through the solar wind, it is estimated it has enough material to last for another 5 billion years. And it is estimated our sun has been in existence for approximately 4.6 billion years!

The photons or sunlight received by the earth is used by plants for energy in synthesising carbon into sugars and releasing oxygen, a process called photosynthesis. We can also use sunlight to create energy using various techniques. The estimated amount of power that the sun deposits on the earth is approximately 1300W/m²; however, due to atmospheric effects, the power is approximately 1000W/m². It is estimated that amount of energy that falls on the earth for an hour could satisfy the entire energy needs of the world for a year!

As this abundant source of energy will be available for a very long time, we should find ways to utilize it. It is free, clean and with our ingenuity, could satisfy all of our energy needs. My next post will discuss some of the ways in which we can use this energy to create usable power.